I would say that it depends on how familiar you are with SwiftUI, and if you are working solo or part of a bigger organisation (or plan to join one).
If you are working by yourself, and only just learning SwiftUI (or any other programming language for that matter) it is a lot to take in all at once. So testing, apart from using the simulator or loading the app on to your own device, is usually left towards the end when you are more comfortable with the language and programming environment.
When you are more familiar with SwiftUI, I would start off by outlining the requirements, and determining the tests (a 'test plan' so to speak) to make sure that you cover all the critical or program sensitive areas, such as proper use of CoreData, etc. The higher level of understanding of the language will mean that you are able to specify, design, implement, test and document code from the requirements and do this for meaningful chunks of the app. This, if you are familiar with the term, is the Agile approach to programming (actually to product and system design more generally), and that after each module or part module is implemented, the resulting product is always useable, even if not all the features are completed yet.
I think that simply put, when you are more confident in using SwiftUI you will probably get in to a cycle of write down your key features, code a bit, test a bit, document it, code a bit, test a bit, document it, . . .
At least this is my thought.